Prior to discussing embodiments of the invention, a general background of the invention will be given, keeping in mind that the recognition of a problem by the applicant may itself comprise at least a portion of the invention. Variable reluctance magnetic transducers such as those used in speed sensing applications are well known. Speed sensing applications generally entail detecting the rotational velocity of a rotating member such as a turbine in an aircraft engine (e.g., to perform speed control) or a wheel on a car (e.g., to perform anti-lock braking). The sensor portion of a variable reluctance speed sensing circuit generally comprises one or more permanent magnets in contact with one or more associated pole pieces formed from a ferrous material. There are a number of ways to arrange the pole pieces and magnets with respect to one another; however a typical technique is to place each pole piece and its respective magnet in an end-to-end relationship. The opposite end of each pole piece, i.e., the end not adjacent to a magnet, is situated near ferrous elements attached to the rotating object of interest, e.g. a speed wheel.
Each pole piece is surrounded by an electrically conductive coil. Changes in magnetic flux through the pole piece will be sensed as electrical changes in the coil, i.e., a voltage level change across the coil or a current change through the coil. Sensing circuitry attached to the coils typically translates the frequency of such electrical changes into a frequency of rotation with respect to the rotating object of interest.
A magnetic field extends from one pole of the magnet, through the pole piece or sensor housing, and then through the air or an adjacent magnetic flux return structure to the other end of the magnet. When a ferrous material is placed near the tip of a pole piece, the reluctance of the magnetic circuit through that pole piece decreases. When this occurs, the strength of the magnetic field in the pole piece increases. Similarly, when the ferrous material is moved away from the pole piece, the strength of the magnetic field in the pole piece decreases.
Changes in magnetic flux within the pole piece induce a corresponding voltage in the coil surrounding the piece. The direction of the induced voltage depends upon the direction of the change in flux. In other words, when the strength of the magnetic field increases, it induces a voltage in the coil in one direction and, when it decreases, it induces a voltage in the opposite direction. Thus it can be appreciated that the approach and departure of one ferrous object to the pole piece induces one cycle of AC voltage.
Since each passage of a ferrous object induces a voltage cycle, the frequency of such cycles can be used to indicate the frequency of passage of ferrous objects. Where the ferrous objects are located on a rotational member such as a speed wheel, this voltage frequency also gives a proportional indication of the speed of rotation of the rotational member.
In order for the induced voltage cycles to be successfully detected for use in speed detection, monitoring, etc., they must be of sufficient magnitude relative to background noise in the signal. The strength of the induced voltage is proportional to the rate of change of magnetic flux in the magnetic circuit, and thus is roughly proportional, up to a point, to the speed of rotation of the body of interest. However, there are other factors that strongly influence the strength of the induced voltage.
For example, the cross-sectional area of the sensor assembly pole piece will constrain the amount of flux passing through the pole piece, such that for smaller diameter pole pieces, the induced voltage may be noticeably decreased. As such, it is difficult to minimize the pole piece footprint while maintaining a sufficient signal-to-noise ratio. The induced voltage is also proportional to the number of turns in the sensor element coil(s). Thus, it is also difficult to minimize the coil diameter while maintaining a sufficient signal-to-noise ratio. The use of multiple sensing elements (i.e. pole/coil assemblies) in a single sensor assembly also limits the coil diameter.
Exacerbating these problems in many environments is the requirement for a large distance between the sensor assembly and the speed wheel due to geometric tolerances, unbalance, clearances, and hot/cold cycles. In addition, the first element to modify the signal is often relatively distant from the sensor assembly itself. Thus the induced signal can decay over the transmission distance prior to use, and additional noise may enter the signal in the leads from the sensor.
An example of such an environment is in aircraft engine monitoring technology, where the variable reluctance sensor is internal to the engine, while the first electronic controller may be located tens of feet away. In addition, in such environments and others, there is little extra space, and thus the sensor assembly and its sensing elements must be compact. Thus, there is a need for a compact sensor element and assembly that can provide a sensed signal of sufficient amplitude and signal-to-noise ratio, even if the sensor assembly is constructed with decreased diameter or greater number of sensing elements relative to traditional sensor assemblies, and/or if the sensor-to-speed wheel distance is increased.